This invention relates to surface wave devices and more particularly to surface wave filters that use cascaded grating resonators to provide a bandpass characteristic.
It has been recognized in the art that useful bandpass filter characteristics can be provided by cascading a plurality of grating resonators. See, for example, the article entitled "Transmission Response of Cascaded Gratings", by H. A. Haus and R. V. Schmidt, IEEE Transactions on Sonics and Ultrasonics, March 1977, pages 94-101. Cascaded grating resonators that are collinearly coupled provide an especially desirable transmission bandpass characteristic in that the characteristic is very flat in the passband and the characteristic has steeper sides than the transmission bandpass of a single resonator. With the resonators collinearly coupled, the excitation source may be placed outside of the cavity thereby eliminating the disturbance that would normally be introduced with an excitation source in the cavity. Unfortunately, this type of structure has no out-of-band rejection for each frequency corresponding to a normalized frequency (.vertline..delta./.kappa..vertline.) greater than one, where .delta. is a measure of the frequency deviation from the Bragg frequency (.beta..sup.2 -.beta..sub.0.sup.2 /2.beta..sub.0) and .kappa. is the coupling coefficient. In fact, for these frequencies that are essentially out-of-band with respect to the desired filter characteristic, the transmission is very close to unity.
The requirement for out-of-band suppression of the transmission characteristic for cascaded grating resonators is recognized in U.S. Pat. No. 4,013,983 to P. Hartemann, issued Mar. 22, 1977. FIG. 5 of the Hartemann patent discloses a proposed solution to this problem in that it discloses a track-changing element to provide the out-of-band suppression. Specifically, the Hartemann patent discloses a right angle reflection grating that couples the input energy from a transducer into a grating resonator. As pointed out in the patent, only the rays that are situated in the stop band of the right angle reflection grating are transmitted to the cascaded grating resonators. At this point in the art, it is virtually impossible to design a right angle reflection grating of the type shown in FIG. 5 of the Hartemann patent to provide the high degree of reflection necessary to introduce low loss within the passband of the resonator, and to provide the degree of suppression necessary to remove the sidelobes normally present in this type of cascaded grating resonator filter.